High temperature li-ion battery cells utilizing boron nitride aerogels and boron nitride nanotubes

ABSTRACT

This disclosure provides systems, methods, and apparatus related to Li-ion batteries. In one aspect an electrolyte structure for use in a battery comprises an electrolyte and an interconnected boron nitride structure disposed in the electrolyte.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 16/869,333,filed May 7, 2020, which is a continuation of U.S. patent applicationSer. No. 15/822,563, filed Nov. 27, 2017, now U.S. Pat. No. 10,686,227,issued Jun. 16, 2020, which claims priority to U.S. Provisional PatentApplication No. 62/428,874, filed Dec. 1, 2016, the entire content ofwhich are hereby incorporated by reference herein.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Contract No.DE-ACO2-05CH11231 awarded by the U.S. Department of Energy, under GrantNo. 1542741 awarded by the National Science Foundation, and under GrantNo. N00014-12-1-1008 awarded by the Office of Naval Research (MUM). Thegovernment has certain rights in this invention.

TECHNICAL FIELD

This disclosure relates generally to batteries and more particularly tolithium-ion batteries.

BACKGROUND

Despite the many advantages and wide commercial usage of lithium-ionbatteries, there remain substantial performance limitations and safetyconcerns. The most serious safety issue involves operating temperaturerange and stability. An increase in internal temperature of the batterycell can lead to internal pressure build up due to an acceleratedreaction rate between the electrolyte and electrodes. Mechanical stressthen damages the electrolyte and separator, resulting in an increasedinternal resistance and thermal runaway. The cell may then short circuitand possibly explode. Since Li-ion batteries self-heat naturally as theyare operated (discharged or charged), developing Li-ion batteries withhigh temperature operation capability is vital for overall safety andreliability. Today's commercial Li-ion battery cells are typicallyunreliable if operated above 60° C., and virtually none can surviveoperation above 100° C.

BRIEF DESCRIPTION OF THE DRAWINGS

Details of one or more embodiments of the subject matter described inthis specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

FIG. 1 shows an example of a schematic illustration of an electrolytestructure including a mesh comprising a plurality of boron nitridenanotubes.

FIG. 2 shows an example of a schematic illustration of an electrolytestructure including a boron nitride aerogel.

FIG. 3 shows an example of a schematic illustration of a lithium ionbattery.

FIGS. 4A and 4B show the electrical response characteristics of Li-ioncells. FIG. 4A shows the impedance of Li-ion cells with differentelectrolyte complexes. FIG. 4B shows the temperature dependence of ionicconductivity of different electrolyte matrices.

FIG. 5 shows the current voltage response of different electrolytecomplexes at 100° C. with a scan of 1 mVs⁻¹.

FIGS. 6A-6D show impedance spectra of the Li-ion cells. The impedance ofthe cells at different temperatures; (FIG. 6A) the cells with BNNTsmodification and (FIG. 6B) the cells with BNAG modification. Theimpedance of the cells with different cycle numbers; (FIG. 6C) the cellswith BNNTs modification and (FIG. 6D) the cells with BNAG modification.

FIGS. 7A and 7B show cyclic voltammograms of Li-ion half cells withLi₄Ti₅O₁₂ at different temperatures: (FIG. 7A) the cells with BNNTs, and(FIG. 7B) the cells with BNAG modifications. FIG. 7C shows theperformance of Li-ion cells with and without BNAG and BNNTsmodification. These cells are studied at elevated temperatures for aLi-ion battery. The cells were tested at 190° C., 145° C., and 100° C.for BNAG, BNNTs, and no filler, respectively.

DETAILED DESCRIPTION

Reference will now be made in detail to some specific examples of theinvention including the best modes contemplated by the inventors forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention.Particular example embodiments of the present invention may beimplemented without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

Various techniques and mechanisms of the present invention willsometimes be described in singular form for clarity. However, it shouldbe noted that some embodiments include multiple iterations of atechnique or multiple instantiations of a mechanism unless notedotherwise.

The terms “about” or “approximate” and the like are synonymous and areused to indicate that the value modified by the term has an understoodrange associated with it, where the range can be ±20%, ±15%, ±10%, ±5%,or ±1%. The term “substantially” is used to indicate that a value isclose to a targeted value, where close can mean, for example, the valueis within 80% of the targeted value, within 90% of the targeted value,within 95% of the targeted value, or within 99% of the targeted value.

High temperature is the bane of Li-ion batteries. Battery cell capacity,efficiency, and cyclability all fade at even slightly elevatedtemperatures. Even worse, high temperature operation can lead tointernal shorting, with consequent thermal runaway and catastrophic(explosive) failure. There are very few existing Li-ion batteryarchitectures that can operate reliably above 60° C., and virtually noneabove 100° C. Described herein is improved thermal stability in Li-ionbatteries by incorporating either a boron nitride aerogel or boronnitride nanotubes into an electrolyte (e.g., a PAN/PMMA basedelectrolyte). Li-ion cells with these electrolyte complexes exhibitexcellent performance and operational stability up to 190° C.

To achieve a Li-ion cell capable of relatively high temperatureoperation (T>60° C.), the liquid electrolyte can be replaced with apolymer electrolyte. This yields other benefits including enhancedprocessability, flexibility, and lighter weight. However, polymerelectrolytes still lose their integrity at temperatures above 100° C.This has led to an examination of reinforced polymer electrolytes. Thereinforcements are largely designed to increase strength, toughness, andflexibility of the polymer. Carbon based materials are promisingcandidates for such modifications.

Interestingly, honeycomb hexagonal boron nitride (h-BN) basednanomaterials can have properties far superior to their carboncounterparts, such as higher oxidation resistance and outstanding hightemperature stability, with no compromise in mechanical strength. h-BNalso is electrically insulating and thermally conductive. These uniqueproperties make h-BN attractive fillers in mechanical andthermally-stable enhanced polymer composites. With a similar sp²-bondconfiguration to graphene, h-BN can be synthesized in analogous lowdimensional forms, such as BN nanotubes (BNNT), BN nanosheets, and BNaerogels (BNAG).

Described herein is an electrolyte structure (also referred to as anelectrolyte complex herein) for Li-ion batteries that is formed byembedding BNAGs or BNNTs into an electrolyte (e.g., a polymer-ionicliquid gel) to increase the electrolyte temperature stability. Theadvantages in chemical inertness, thermal stability, and high surfacearea of BNAG and BNNTs elevate the reliable and safe battery operationtemperature to 190° C., while still maintaining high capacity andcyclability.

In some embodiments, an electrolyte structure for a Li-ion batterycomprises an electrolyte with an interconnected boron nitride structuredisposed therein. In some embodiments, the interconnected boron nitridestructure comprises hexagonal boron nitride. In some embodiments, theelectrolyte comprises an organic electrolyte. In some embodiments, theorganic electrolyte comprises a polymer electrolyte. In someembodiments, the polymer electrolyte comprises a polymer mixturecomprising an ionic conducting polymer comprising a polymer mixed with alithium salt. In some embodiments, the polymer is selected from a groupconsisting of poly(propylene oxide) (PPO), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(vinylidene fluoride) (PVDF), andpoly(methyl methacrylate) (PMMA). Polymer electrolytes generally areable to function at higher temperature compared to some organicelectrolytes. In some embodiments, the organic electrolyte is selectedfrom a group consisting of an organic ester, an organic ether, andethylene carbonate (EC). In some embodiments, the organic electrolyte iscoupled with a linear carbonate co-solvent, such as dimethyl carbonate(DMC), diethyl carbonate (DEC), or ethylmethyl carbonate (EMC), forexample. In some embodiments, the organic electrolyte is doped withlithium hexafluorophosphate (LiPF₆).

In some embodiments, the interconnected boron nitride structurecomprises a mesh comprising a plurality of boron nitride nanotubes. Amesh is a network (e.g., a group of interconnected objects) of theplurality of boron nitride nanotubes. Individual boron nitride nanotubesare in contact with other boron nitride nanotubes in the mesh. Such amesh may be also be considered a sheet of material comprising theplurality of boron nitride nanotubes.

As shown in FIG. 1, in some embodiments, an electrolyte structure 100includes an electrolyte 110 with a plurality of boron nitride nanotubes(one boron nitride nanotube is labelled 105) disposed in the electrolyte110. The electrolyte structure 100 may be considered a composite. Insome embodiments, boron nitride nanotubes of the plurality boron nitridenanotubes have a length of about 30 nanometers (nm) to 1 micron. In someembodiments, the boron nitride nanotubes of the plurality boron nitridenanotubes have a diameter of about 1 nm to 20 nm or about 5 nm to 20 nm.In some embodiments, the mesh has a thickness of about 250 nm to 1.5microns, or about 500 nm to 1 micron. In some embodiments, the mesh hasa volume fraction of about 2% to 50% in the electrolyte structure 100.

In some embodiments, the interconnected boron nitride structurecomprises a boron nitride aerogel. FIG. 2 shows an example of aschematic illustration of electrolyte structure 205 including anaerogel. An aerogel is a porous ultralight material derived from a gel.An aerogel is a gel in which the liquid component of the gel has beenreplaced with a gas (e.g., air). In some embodiments, the boron nitrideaerogel has pore sizes of less than about 100 nm. In some embodiments,the boron nitride aerogel has pore sizes of about 2 nm to 100 nm. Insome embodiments, the boron nitride aerogel has a thickness of about 250nm to 1.5 microns, about 500 nm to 1 micron, or about 500 nm to 25microns. In some embodiments, the boron nitride aerogel has a volumefraction of about 2% to 50% in the electrolyte structure 205. In someembodiments, the boron nitride aerogel has a density as low as 0.6mg/cm³. In some embodiments, the boron nitride aerogel has a specificsurface area as high as 1050 m²/g.

In some embodiments, a plurality of titanium dioxide (TiO₂)nanoparticles is disposed in the electrolyte. In some embodiments,titanium dioxide nanoparticles of the plurality of titanium dioxidenanoparticles have a diameter of about 1 nm to 100 nm. In someembodiments, the plurality of titanium dioxide nanoparticles has avolume fraction of about 5% to 30% in the electrolyte structure.Titanium dioxide nanoparticles can improve that charge/dischargeperformance of a cell and can enhance ion concentration in a cell. Othernanoparticles can also be added to the electrolyte to achieve similarperformance enhancements. Such nanoparticles include germanium-basednanoparticles (e.g., GeS), cobalt-iron oxide based nanoparticles (e.g.,Co₃O₄, Fe₃O₄, CoFe₂O₄), and molybdenum-based nanoparticles (e.g., MoO₃).

Benefits of an interconnected boron nitride structure disposed in anelectrolyte incorporated in a Li-ion battery include higher temperatureoperation of the Li-ion battery. In some embodiments, a Li-ion batteryincluding an interconnected boron nitride structure disposed in anelectrolyte is operable up to temperatures of about 190° C. In someembodiments, an interconnected boron nitride structure (e.g., a meshcomprising nanotubes or an aerogel) serves to distribute heat moreevenly in the Li-ion battery compared to boron nitride nanoflakes orboron nitride nanoparticles (both of which do not form interconnectedstructures) due to the higher thermal conductivity of the interconnectedstructure.

In some embodiments, when localized thermal runaway occurs in a Li-ionbattery, the interconnected boron nitride structure can aid inpreventing contact between the anode and cathode and aid in preventingion conduction in the localized region of the thermal runaway. Further,due to the high thermal conductivity of boron nitride, heat isdissipated quickly at the localized regions. While not wanting to bebound by any theory, in localized regions that reach high temperatures,it is believed that the interconnected boron nitride structure“collapses” and the porosity of the interconnected boron nitridestructure decreases in the localized region of the thermal runaway. Thedecrease in the porosity of the interconnected boron nitride may be dueto high temperatures stressing cross-links in the boron nitride aerogelwhich causes a collapse in the region.

FIG. 3 shows an example of a schematic illustration of a lithium ionbattery. As shown in FIG. 3, a lithium ion battery 300 includes an anode305, a cathode 310, and an electrolyte structure 315. When in operation,the lithium ion battery 300 provides current to a load. The electrolytestructure 315 is in contact with the anode 305 and the cathode 310. Theelectrolyte structure 315 comprises any of the electrolyte structures(e.g., an electrolyte having a boron nitride aerogel or a plurality ofboron nitride nanotubes disposed therein) described herein.

In some embodiments, the anode 305 comprises lithium titanate(Li₄Ti₅O₁₂) or graphite. In some embodiments, the cathode 310 compriseslithium-rich manganese oxide (Li_(1+x)Mn_(2−x)O₄) or lithium cobaltoxide (LiCoO₂). In some embodiments, the polymer membrane is a cationconductor and is not electrically conductive. In some embodiments, theelectrolyte structure is about 250 nm to 1.5 microns thick, or about 500nm to 1 micron thick.

In some embodiments, a method of fabricating an electrolyte structureincludes providing an interconnected boron nitride structure. Methods offabricating boron nitride nanotubes are described in U.S. Pat. No.9,394,632, titled “Method and device to synthesize boron nitridenanotubes and related nanoparticles,” and U.S. patent application Ser.No. 15/321,177, titled “System and methods for fabricating boron nitridenanostructures,” filed Jun. 24, 2015, both of which are hereinincorporated by reference. After fabrication of boron nitride nanotubes,the nanotubes may be cleaned in a solvent. Filter paper may be used toseparate the nanotubes from the solvent. A mesh is formed by thenanotubes drying on the filter paper. Methods of fabricating boronnitride aerogels are described in U.S. Pat. No. 9,611,146, titled“Crystalline boron nitride aerogels,” which is herein incorporated byreference.

In some embodiments, the interconnected boron nitride structure is thencontacted with an electrolyte. The electrolyte flows into theinterconnected boron nitride structure. In some embodiments, theinterconnected boron nitride structure is immersed in the electrolyte.In some embodiments, operations with the electrolyte are performed invacuum environment or an inert (e.g., argon) environment. After theelectrode structure is fabricated, the Li-ion battery is assembled, withthe electrode structure being positioned between an anode and a cathode.

In some embodiments, an interconnected boron nitride structure ispositioned between and in contact with an anode and a cathode. Forexample, a sheet of a boron nitride aerogel may be positioned betweenand in contact with an anode and a cathode. As another example, a meshcomprising a plurality of boron nitride nanotubes may be positionedbetween and in contact with an anode and a cathode. The electrolyte isthen contacted with or injected into the interconnected boron nitridestructure. In some embodiments, operations with the electrolyte areperformed in vacuum environment or an inert (e.g., argon) environment.

In some embodiments, the electrolyte is at an elevated temperature whenit comes into contact with the interconnected boron nitride structure.When the electrolyte cools (i.e., cools below the glass transitiontemperature), it solidifies. This forms a solid state ion conductingstructure.

EXAMPLE

The following examples are intended to be examples of the embodimentsdisclosed herein, and are not intended to be limiting.

Electrolyte complexes were prepared by mixing either BNNTs or BNAG withpolymethylmethacrylate (PMMA) and polyacrylonitrile (PAN) (1:2:9 weightpercentages) dissolved in anisole. Subsequently this mixture wasincorporated into the ionic liquid PYR₁₄-TFSI-LiTF SI (PYR₁₄:N-methyl-N-butylpyrrolidinium, TFSI: bis(trifluoromethylsufonyl)imide).In some instances, TiO₂ nanoparticles were added to the solution inorder to enhance the concentration of ions in the cell. The most common,commercially available electrodes were used for the anode and cathode:lithium titanate (Li₄Ti₅O₁₂) as the anode and lithium-rich manganeseoxide (Li_(1+x)Mn_(2−x)O₄) as the cathode. The Li₄Ti₅O₁₂ is a highperformance, zero strain electrode with a lithium insertion potential of1.55 V. Li_(1+x)Mn_(2−x)O₄ has excellent compatibility with Li₄Ti₅O₁₂due to the similar spinel structure. Further details about materialssynthesis and sample preparation can be found below.

The cells were first characterized based on their ionic conductivitiesand concentration. One potential concern for BN filler materials isthat, though they can be used to form more mechanically stable polymerelectrolytes, their insulating properties can lower ion concentrationsand inhibit their motion across the cell. The lithium ion transferencenumber (t_(Li) ⁺) readily determines the ion concentration gradient inthe electrolyte during the charge and discharge process and is relatedto the number of moles of lithium ions passing through the electrolyteper unit capacitance. In order to determine the transference number forthe BNNT and BNAG samples, lithium symmetric cells (Li/electrolyte/Li)were prepared and a previously described dc polarization measurement wasused. Results of these measurements are reported in Table 1.

TABLE 1 Lithium transference number for different electrolyte gelcompositions, at selected temperatures. Solvent t_(Li) ⁺ t_(Li) ⁺ t_(Li)⁺ t_(Li) ⁺ Electrolyte gel ratio (24° C.) (60° C.) (80° C.) (100° C.)PAN-PMMA 2:9 0.652 0.691 0.713 0.774 BNAG-PAN-PMMA 1:2:9 0.482 0.5010.532 0.593 BNNT-PAN-PMMA 1:2:9 0.392 0.412 0.443 0.501 TiO₂-BNAG-PAN-1:6:2:7 0.583 0.596 0.621 0.641 PMMA TiO₂-BNNT-PAN- 1:6:2:7 0.471 0.4870.505 0.549 PMMA

In general, values of t_(Li) ⁺ approaching unity are desirable to assuregradient homogeneity across the electrolyte. However, in most of thecases, adding a filler such as BNAG or BNNT into the electrolyte systemcan produce an internal charge. This internal charge can create anadditional polarization loss due to a possible ion exchange between thecomposite and the ionic liquid). Thus transference numbers less than oneare typical for composite electrolytes. For example, PAN/PMMA basedionic gel systems have transference numbers on the order of 0.5 -0.7. Asshown in Table 1, t_(Li) ⁺ for the electrolytes without filler, withBNAG filler, and with BNNT filler are 0.65, 0.48, and 0.39 at roomtemperature, respectively. Importantly, it was found that the iontransference number increases with increasing temperature (respectivevalues become 0.77, 0.59, and 0.50 at 100° C.). Furthermore, t_(Li) ⁺for the BNAG and BNNT containing electrolytes is increased by 20% byadding a small amount of TiO2 nanoparticles, yielding 0.58 for TiO₂/BNAGaddition and 0.47 for TiO₂/BNNT addition at room temperature, and 0.64and 0.55 at 100° C., respectively. These t_(Li) ⁺ values are acceptable.

The thermal stability of the different electrolyte compositions wasinvestigated by solution impedance and ionic conductivity measurementsat different temperatures. FIG. 4A shows the ac impedance of theelectrolytes as a function of temperature. Sharp increases are observedat 120° C. for unfilled PAN-PMMA, and at 145° C. and 190° C. forelectrolyte filled with BNNTs and BNAG, respectively. Additions of TiO₂have only a minor effect on the impedance thresholds for the BNNT andBNAG samples. The rapid increase in impedance signals failure of theelectrolyte complex in blocking active ionic transport. Thereafter theimpedance grows quickly with increasing temperature, which causes severepressure build up in the cells due to gas generation. This causes theelectrolyte complex to fail entirely and short circuit the cellinternally, whereupon the impedance drops precipitously.

The improvements in thermal stability for the BNNT and BNAG fillerssuggested by the impedance threshold data of FIG. 4A are even moredramatic than the figure implies. In particular, the electrolyte mustwithstand repeated cycling. At 120° C., it was found that unfilledPAN-PMMA is prone to failure after only a few cycles, whereas the BNNTand BNAG filled cells show stable behavior over at least 500 cycles at145° C. and 190° C., respectively.

FIG. 4B shows the relationship of ionic conductivity of the electrolytewith respect to temperature. As expected, the electrolyte with onlyPAN/PMMA has the highest ionic conductivity, but the electrolytecomposites with BNAG and BNNTs also exhibit good conductivity. Thereason the cells with BNAG have higher performance than the cells withBNNTs is the ability of BNAG to absorb more of the ionic solution due toits higher specific surface area. Adding TiO₂ nanoparticles does notappreciably affect the temperature limit but does slightly increaseionic conductivity. The data of FIGS. 4A and 4B indicate that polymerreinforcements with BNAG and BNNTs increase the strength and structuralintegrity of the polymer. With these fillers, the polymer can surviveelevated temperatures and elevated internal pressures.

The electrochemical stability and anodic decomposition of the cells wastested by measuring the current-voltage response using linear voltagesweeps at 100° C. As shown in FIG. 5, the BNNT electrolyte complex hasan anodic stability up to 5.05V vs. Li at this elevated temperature. Thedecomposition voltage of the BNAG electrolyte complex extends above 5.4Vvs. Li, indicating additional electrolyte absorption ability of BNAGwhich further improves electrochemical stability of the electrolyte. Thecells with TiO₂ nanoparticles show slightly more improved anodicstability. This nested electrolyte design shows good anodic stability,which makes it compatible with negative electrode systems (e.g., carbonor silicon based), essential to high-voltage batteries for high energyapplications.

The impedance of the cells at different temperature and cycle number wasmeasured by alternating-current impedance spectroscopy (FIGS. 6A-6D).With this method, the interface between the electrode and the polymerelectrolyte is directly probed. The resistances can be broken down intothree regimes: data at high frequencies are related to ionicconductivity, at medium frequencies to the charge transfer betweenelectrodes, and at lower frequencies to diffusion of lithium ions in thecathode. When temperature increases, data become more intertwined andindistinguishable because of higher chemical reactivity. The thermalenergy flowing in the cell provides more kinetic energy to the ionswhich causes the ions, at interfaces, to begin to move much fastertoward the electrodes. Thus, ionic conductivity increases, diffusionprocesses become predominant, and the impedance decreases. Cells withBNNTs exhibit higher impedance at elevated temperatures than cellscontaining BNAG, which indicates that ionic diffusion is more efficientthrough the BNAG electrolyte complex.

In order to establish overall cell performance characteristics, thevariation of impedance with the cycle number (FIGS. 6C and 6D) wasdetermined. As cycle number increases the data at the middle frequencyregion increase due to capacity fading. The porosity of the cathodeslowly decreases due to accumulation of residual reactants at theinterface. The cells with BNAG and BNNTs have similar characteristicsthroughout the first 20 cycles. However, at later cycles, the cells withBNAG modification exhibit lower resistance and have better overallcharge-transfer than cells with BNNTs, which indicates the BNAGelectrolyte complex has better interface stability which improveselectrolyte retention characteristics. The cells with no fillers exhibita similar trace at the first two cycles but then show a marked increasein resistance. The cells with TiO₂ nanoparticles exhibit better behaviorat the middle frequency region, as expected, but poor characteristics inthe low frequency region due to poor interstitial diffusion.

FIGS. 7A and 7B show cyclic voltammograms of cells with BNAG and BNNTsmodifications at different temperatures. FIG. 7C shows capacity vs.cycle number. These figures provide a clear indication of the importanceof polymer reinforcement on cell performance. The symmetrical feature inthe voltammograms corresponds to lithium insertion and extraction. Thesmooth and highly symmetric peaks indicate that the lithium insertionand extraction are reversible and no other reaction is involved duringthese processes. Increasing temperature results in an enhanced peakcurrent and peak separation. The energy is also shifted due to thestronger diffusion process by 30 mV and 40 mV for BNNTs and BNAG,respectively. This behavior results in inequality of anodic and cathodiccurrent behavior. But, as seen in FIGS. 7A-7C, cells are very stableeven at high temperatures. These cells are very stable over 500 cyclesat elevated temperatures (145° C. and 190° C. for BNNTs and BNAG,respectively), while the cells with no filler decline steadily after 5cycles (FIG. 7C). The cells with TiO₂ yield better capacity at roomtemperature up to 100 cycles, but at elevated temperatures the cellssuffer severe capacity degradation after the first 20 cycles. Thebattery power loss at elevated temperatures could be because of theparasitic reactions among the solvents and TiO₂ interface.

Details of Electrolyte and Electrode Preparation: The lithium titanate(Li₄Ti₅O₁₂), spinel nanopowder, (<200 nm particle size (BET))>99%) anodewas prepared by casting a slurry of 80 wt % active material, 15 wt %conductive reagent (acetylene black), and 5 wt % poly-vinylidenedifluoride (PVDF) binder on to aluminum (Al) foil. Lithium-richmanganese oxide (LiMnO₄) powder was mixed with acetylene black and PVDFat a rate of 85:10:5 (wt %) and coated on Al foil. Then the electrodeswas dried at 100° C. in vacuum overnight. The loading ratio between theanode and cathode was ˜2:5 by weight. PAN/PMMA electrolyte was preparedby a polymer solution casting method by reinforcing with BNNTs or BNAGat a rate 1:2:9 (wt %). This mixture was then activated with 1M ionicliquid PYR₁₄-TFSI-LiTFSI. This electrolyte mixture was then depositedonto the electrode surface as an individual solid layer by thickness of150 microns to 250 microns.

Details of Electrochemical Characterization: The lithium transferencenumber was measured in lithium symmetric cells (Li/electrolyte/Li) atroom temperature with a dc polarization method. The electro chemicalimpedance spectroscopy, cyclic voltammetry, galvanostatic chargedischarge analysis, and the ionic conductivity of the cells were takenby using computer controlled potentiostat-galvanostat and impedanceanalyzer at the frequency range from 0.1 Hz to 1000 kHz.

CONCLUSION

In conclusion, battery cells that can operate at high temperatures witha long cycle lifetime have been developed. Electrolyte complexes wereprepared by embedding polymer electrolytes with BNAG and BNNTs. Thecells with the BNAG modification display good electrolyte wettabilitywith excellent ionic conductivity and transference number. Thesemodifications also improve overall thermal protection of the batterycells. The cells with BNAG modification can operate up to 190° C. withover 500 cycles, while BNNTs extend safe and reliable operation up to145° C. It was demonstrated that BNNT and BNAG modifications enable highvoltage, high performance Li-ion batteries with outstanding performanceand attractive safety features towards explosion proof batteries.

In the foregoing specification, the invention has been described withreference to specific embodiments. However, one of ordinary skill in theart appreciates that various modifications and changes can be madewithout departing from the scope of the invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofinvention.

1. (canceled)
 2. An electrolyte structure for use in a batterycomprising: an electrolyte comprising a polymer mixed with a lithiumsalt; and an interconnected boron nitride mesh structure in theelectrolyte and comprising a plurality of boron nitride nanotubes. 3.The electrolyte structure of claim 2, wherein the electrolyte structurefurther comprises a boron nitride aerogel.
 4. The electrolyte structureof claim 2, wherein the polymer comprises one or more of poly(propyleneoxide) (PPO), poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN),poly(vinylidene fluoride) (PVDF), or poly(methyl methacrylate) (PMMA).5. The electrolyte structure of claim 2, wherein the electrolyte furthercomprises a solid-state ion conducting structure.
 6. The electrolytestructure of claim 2, further comprising: a plurality of titaniumdioxide nanoparticles disposed in the electrolyte.
 7. The electrolytestructure of claim 6, wherein the plurality of titanium dioxidenanoparticles has a volume fraction of about 5% to 30% in theelectrolyte structure.
 8. The electrolyte structure of claim 2, whereinboron nitride nanotubes of the plurality of boron nitride nanotubes havea length of about 30 nanometers to 1 micron and have a diameter of about1 nanometers to 20 nanometers.
 9. The electrolyte structure of claim 2,wherein the interconnected boron nitride mesh structure has a thicknessof about 250 nanometers to 1.5 microns.
 10. The electrolyte structure ofclaim 2, wherein the plurality of boron nitride nanotubes comprisehexagonal boron nitride (h-BN).
 11. The electrolyte structure of claim2, wherein the electrolyte is operationally stable up to 190° C.
 12. Abattery comprising: an anode; a cathode; and an electrolyte structure,the electrolyte structure being in contact with the cathode and with theanode, the electrolyte structure comprising: an electrolyte comprising apolymer mixed with a lithium salt; and an interconnected boron nitridemesh structure in the electrolyte and comprising a plurality of boronnitride nanotubes.
 13. The battery of claim 12, wherein the battery isoperable up to temperatures of about 190° C.
 14. The battery of claim12, wherein the anode comprises lithium titanate (Li₄Ti₅O₁₂) orgraphite.
 15. The battery of claim 12, wherein the cathode compriseslithium-rich manganese oxide or lithium cobalt oxide (LiCoO₂).
 16. Thebattery of claim 12, wherein the interconnected boron nitride meshstructure contacts the anode and the cathode.
 17. The battery of claim12, wherein the electrolyte structure further comprises a boron nitrideaerogel.
 18. The battery of claim 12, wherein the polymer comprises oneor more of poly(propylene oxide) (PPO), poly(ethylene oxide) (PEO),poly(acrylonitrile) (PAN), poly(vinylidene fluoride) (PVDF), orpoly(methyl methacrylate) (PMMA).
 19. The battery of claim 12, whereinthe electrolyte further comprises a solid-state ion conductingstructure.
 20. The battery of claim 12, wherein the plurality of boronnitride nanotubes comprise hexagonal boron nitride (h-BN).
 21. Thebattery of claim 12, wherein the electrolyte comprises one or more ofgermanium-based nanoparticles, cobalt-iron oxide based nanoparticles, ormolybdenum-based nanoparticles.